Exhaust gas recirculation (EGR) is employed in internal combustion engines to control NOx emissions. For example, diesel engines typically operate with a lean air/fuel ratio globally, but during combustion a flame itself burns locally at or near a stoichiometric condition. EGR gas is introduced as a diluent that effectively reduces flame temperatures, and correspondingly reduces nitrogen oxide (NOx) emissions. While adding EGR gas reduces NOx, the reduced flame temperatures can result in an increase in particulate matter (PM). Thus, EGR gas needs to be accurately controlled in order to control both NOx and PM emissions.
In one approach, all exhaust from one or more cylinders is routed back into the intake manifold. These cylinders are referred to as donor cylinders. On the other hand, all exhaust from one or more other cylinders is routed to an exhaust pipe. These cylinders are referred to as non-donor cylinders. In such a configuration, EGR rate is typically controlled through operation of a valve that is located in an EGR passage between an exhaust manifold of the donor cylinders and the intake manifold. The valve position is controlled to vary a mass flow rate of EGR gas provided to the intake manifold.
However, the inventors herein have identified issues with the above described approach. For example, by controlling EGR composition through control of EGR mass flow rate, an oxygen concentration in the intake manifold may be controlled with less accuracy or looser tolerances that result in greater NOx emissions.